It is known that one of the means adopted for decreasing the amount of unburned components in the exhaust gases of internal combustion engines is to encourage an additional combustion of said components downstream of the engine exhaust valves (that is, after that the exhaust gases have left the cylinder, but in the interior of the exhaust system and upstream of the point at which the exhaust gases reach the atmosphere).
Such an additional combustion can take place by resorting to oxygen which is possibly still contained in the exhaust gases, or, as an alternative, by exploiting air which is specially fed into the exhaust system.
It is likewise known that such a combustion can take place only if the temperature of the exhaust gases is above a certain magnitude, so that it becomes profitable, if not imperative, to prevent the cooling of the exhaust gases not only in the zone of the exhaust system in which the additional combustion should take place, but also in the section located between the engine head and said post-combustion area.
It is known, moreover, that in many engines, more particularly in motor-car engines, the exhaust system is so designed as to utilize, in order to improve the volumetric efficiency, the pressure pulsations in the interior of the exhaust system so as to increase the specific horsepower of the engine: in such a case, the optimum cylinder filling is generally obtained by keeping separate from each other, along a certain length, the exhaust ducts communicating with the individual cylinders and emerging from the engine head. Thus, the portion of the exhaust system which lies in the neighborhood of the engine head has, in such engines, a considerable branching off, with a very high external surface which favors the dispersion of heat and the cooling of exhaust gases.
Whenever it is desirable, on engines of the kind referred to above, to obtain an additional combustion of the exhaust gases as outlined in the foregoing, such heat dispersion must be limited as far as practicable, by resorting to heat-insulation: this problem, however, is not easily solved, on account of the fact that the heat insulation assembly should withstand very high temperatures (especially when the engine displays its maximum horsepower), mechanical fatigue stresses due to vibrations, and mechanical stresses due to different thermal expansion coefficients: in addition, the heat insulation must be susceptible of mass production at economically acceptable costs.